TNF‐α, inefficient by itself, potentiates IL‐1β‐induced PGHS‐2 expression in human pulmonary microvascular endothelial cells: requirement of NF‐κB and p38 MAPK pathways

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Abstract
1: Prostaglandin H synthase-2 (PGHS-2), is an inducible enzyme involved in various inflammatory responses. We established here that interleukin-1beta (IL-1beta) but not tumour necrosis factor-alpha (TNF-alpha) increased its expression in human pulmonary microvascular endothelial cells (HPMEC). However, associated with IL-1beta, TNF-alpha greatly potentiated this enzyme induction. 2: Although unable to induce PGHS-2 expression by itself, TNF-alpha promoted a similar transcription nuclear factor-kappaB (NF-kappaB) activation to IL-1beta. This effect was more pronounced when cells were co-exposed to both cytokines. HPMEC pre-treatment with MG-132, a proteasome inhibitor, prevented NF-kappaB activation as well as more distal signalling response, indicating that NF-kappaB activation is required but not sufficient for PGHS-2 expression. 3: Both IL-1beta and TNF-alpha failed to activate c-Jun NH2-terminal kinase (JNK). In addition, PD98059, a p42/44 mitogen-activated protein kinase (MAPK) phosphorylation inhibitor, did not decrease PGHS-2 expression. However, SB 203580, a p38 MAPK inhibitor, suppressed PGHS-2 induction by IL-1beta alone or combined with TNF-alpha, demonstrating that p38 MAPK but not p42/44 MAPK or JNK cascades are required for PGHS-2 up-regulation. 4: Finally, TNF-alpha, unlike IL-1beta, was unable to promote p38 MAPK phosphorylation, indicating that the failure of TNF-alpha to induce PGHS-2 expression is linked, at least in part, to its inability to activate p38 MAPK signalling pathway. Altogether, these data enhanced our understanding of PGHS-2 regulation in HPMEC and emphasize the heterogeneity of cellular responses to proinflammatory cytokines.

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